The first problem I ran into was that it wouldn't boot from the
install media. This was "Secure Boot" in action. I had to figure out
how to get into the BIOS settings page (ESC while booting), disable
Secure Boot, reboot, enable Legacy Boot, then boot into the install.

The second problem was that after the install process completed,
the BIOS claimed it couldn't find anything to boot. I asked around
and guessed the problem was the HP BIOS couldn't deal with GPT,
so I redid the install with old-style BSD Labels partitioning.
That was more complicated than it should have been because the
10.0 installer switched from sysinstall to bsdinstall and the
partitioning page is just different enough to confuse old-timers
like me. I ended up doing something ridiculous: starting a 9.2
install, partitioning using good old sysinstall, quitting, starting
the 10.0 install, and skipping the partitioning step. Anyway,
it worked.

After that things went pretty smoothly. I installed rsync from
ports, Xorg and Firefox from packages, and everything is running
pretty smoothly. My remaining issues list:

Figure out how to update the ports tree. I think this is something
like "portsnap fetch ; portsnap update" as root, except not from
a cron job.

Figure out how to update the packages tree too.

There's no bell. Sound works, aside from that. Bell is also
absent in the raw console, and even at boot time, so this is
something pretty basic.

The HP keyboard kind of sux0rs, I will probably get a new one.
The mouse is ok, hardware-wise. However,

The mouse is too sensitive for me by a fairly large factor,
at least 2x maybe even 3x. None of the various things I've tried
for slowing it down have worked - they don't seem to change the
pointer speed at all. This includes xset and a couple of options
in xorg.conf.

There's a trivial bug with xterm's cursor. It's supposed to be
a hollow rectangle when the window does not have focus and a solid
rectangle when it does. However, when the mouse pointer goes directly
from Firefox to an xterm, the cursor stays hollow. The xterm still
gets focus, but the cursor does not change. I used xev to look at
what events the xterm is getting and found that in the broken
case, where the cursor stays hollow, xterm gets a FocusIn event,
while in the correct case it does not. Kind of the opposite of what
I was expecting. Google shows some folks ran into a similar-sounding
issue with xemacs instead of xterm a couple years ago, and had to
change some xemacs code to fix it.

Finally, in FreeBSD 10.0 gcc is no longer installed by default,
it uses clang instead, so I have been making sure all my software
builds cleanly under clang. I have a couple hundred packages to
go through so this will take a while. The first thing I did is edit
all the Makefiles so instead of calling gcc explicitly, they call
cc, which is a front-end that calls gcc or clang as appropriate. So the
packages will build unmodified on either variety of system. While I use
very few of those hundreds of software packages on my desktop machine,
starting the clangification process now will make things easier when I
switch my server to 10.0 later.